1. Field of the Invention
The present invention relates to an apparatus for determining the authenticity of documents containing piezo electric materials.
2. Summary of the Prior Art
It is known by way of U.S. Pat. No. 4,792,667 to take advantage of the pyroelectric and piezoelectric characteristics of polarized polymers in order to determine whether an authentic and un-altered document has been presented. A polymeric material formed as a pre-poled film may be affixed to the surface of a document by an adhesive or may be applied as a coating to the surface of the document. If not pre-poled, the polymeric material may be polarized after being placed on the document. A testing apparatus is also disclosed therein that physically stimulates the polymeric material and detects a level of current flow that is indicative of polarized material.
The devices disclosed, use either a heating element to effect a temperature change in the material or a device to impart a physical impact thereupon, whereby as a result of each stimulus a current is generated that may be detected in order to determine whether the piezo-electric structure is intact. A problem with the afore going is that the testing is relatively slow in nature with respect to the thermal excitation and a complicated mechanical device is required where the mechanical actuation is used, thereby making these known structures impractical for the high volumes necessary to verify documents. Another problem is that the output from the piezoelectric material that these devices choose to measure is current, which is the rate of change of charge being given off by the material as it is changing in response to a particular input. This is a problem as piezo film typically offers very high voltage sensitivity, but relatively poor charge sensitivity, whereby the current output is low. Apparent from the mentioned disclosure, is that the heat transport occurs by conduction or convection. In both cases the rate of change in temperature seen by the piezoelectric material will be relatively low and the resulting mechanical expansion which produces the net current output will be slow so that the output is low as indicated by reference to utilization of a picroampeter, thereby providing a difficult to measure response that as a result of the construction of the sample and associated variations therein will also vary.